Photodegradation of chloromycetin in aqueous solutions: kinetics and influencing factors

Abstract

This study of photodegradation of the antibiotic chloromycetin (Cm) in aqueous solution by direct and indirect photolysis included photolysis under UV-C light (k = 254 nm) and photo-oxidation under UV-vis light (k C 365 nm) in the presence of iron and humic acid. The factors affecting Cm degradation were studied and are described in detail, including initial pH, ionic strength and initial concentrations of iron and humic acid. Results showed that a degradation efficiency up to 90% was achieved by direct photolysis of Cm at pH 5-7 and the calculated quantum yield was 0.084. Higher salt content (NaCl, 0.01-0.5 M) was found to benefit direct photolysis. Indirect photolysis of Cm in the presence of iron(III) formed OH • radicals at pH * 3. Under UV-vis light, increased pH resulted in a significant decrease in the efficiency of indirect photolysis. Direct and indirect photolysis reactions both followed a pseudo first-order kinetic law. Humic acid tended to inhibit the photodegradation of Cm under the conditions of this work, implying that photosensitization of humic acid did not play any role in the photodegradation.

Chloramphenicol

“…Aromatic nitro compounds, however, are well known for their insolubility, particularly in aqueous solutions, and chloramphenicol is no exception -it has low aqueous solubility ($2.5 g/L at 25 C and neutral pH), with its lowest solubility occurring at pH 5.5-7.0 (i.e. Chloramphenicol itself is stable, with a shelf life of several years for the crystalline form, and it retains 100% of its activity for 24 hours in solution over a pH range of 1-9.5 (Bartz, 1948); however, over a longer time period, it degrades in solution due to exposure to light (Zhou et al, 2010), heat (Franje et al, 2010), and different pHs (Boer and Pijnenburg, 1983;Zhou et al, 2010). Amide NH groups are usually considered to be neutral, but that of chloramphenicol is significantly more acidic than most, due to the electronegative influence of the two chlorine atoms causing greater than usual electron flow from the amide N to the carbonyl group.…”